1. Field of the Invention
The invention relates to testing digital relays used in power transmission systems, and more particularly, to an interface circuit for converting the high current signal outputs of test equipment to low voltage signals appropriate for use by digital relays.
2. Description of the Related Art
In modern high voltage power transmission systems, various types of protective equipment are used to respond to system faults such as short circuits. The protective equipment typically comprises three types of elements: transducers, relays and circuit breakers. When a fault is detected, the relays signal the appropriate circuit breakers to trip and isolate the faulty equipment. This protects not only the faulty equipment from further damage, but it also minimizes damage to the rest of the power transmission system.
The transducers are voltage and current transformers which transform high voltages and currents to a more manageable level for use by the relays. The output currents provided by these current transducers are referred to as secondary currents. The standard for the secondary current is five amperes in the United States, and one ampere in most European countries. Relays typically used are electro-mechanical relays that are responsive to the ratio of voltages and currents at the relay location.
Digital relays have been increasingly used in power systems due to their relative low cost and improved versatility. The digital relay includes a microcontroller or microprocessor to control the functions of the relay. With the advent of digital relays, transducers have been developed which transform the high-voltage power system signals to signals in the hundreds of millivolts RMS range. One such transducer is the magneto-optical current transducer (MOCT), whose output is at a voltage level of 200 mV RMS, which corresponds to the five-ampere output of a conventional current transducer. The low voltage signal output of the optical current transducer is provided to a digital relay, which samples the low voltage signal to determine the appropriate action to take.
One problem that has arisen with the use of such low voltage digital relays is the unavailability of proper test equipment. Most test equipment in use provides high current signal outputs for use with conventional electro-mechanical relays. Consequently, such test equipment cannot be used directly with the low voltage digital relays. However, purchasing new test equipment to provide the appropriate signal outputs would be very expensive. Thus, one solution that has been implemented is to use transformers as interface devices between the test equipment and the digital relay to convert the high current test signals to the appropriate voltage for use with the digital relay. However, transformers are passive devices that cannot easily be recalibrated to provide different voltage levels required by different digital relays.